Importance of Asp1191 for tyrosine kinase activity of the insulin receptor: functional difference of universally conserved Asp between tyrosine kinase and c-AMP dependent serine/threonine protein kinase.

Asp1191 of the tyrosine kinase domain in the insulin receptor is located in the almost perfectly conserved Ser-Asp-X-Trp motif of all tyrosine kinases, and the function of the motif has not been clarified. In this motif, Asp1191 is universally conserved in all protein kinases and corresponds to Asp220 in catalytic subunit of c-AMP dependent protein kinase. However, while other universally conserved amino acid residues are involved in catalysis, only Asp220 is not. To determine whether this motif was not also involved in catalysis, Asp1191 was replaced either by alanine, asparagine, or glutamic acid and Met1192 was replaced by valine. Asp1191-->Ala, Asn or Glu receptor lacked tyrosine kinase activity, while Met1192-->Val receptor showed normal kinase activity. These mutant receptors had no effect on insulin binding. These results suggested that the universally conserved Asp1191 in all protein kinases, which was not required for catalytic action of c-AMP dependent protein kinase, was essential for the tyrosine kinase activity of the insulin receptor.